IR or thermal imaging systems typically use a plurality of thermal sensors to detect IR radiation and may produce an image capable of being visualized by the human eye. For example, thermal imaging systems typically detect thermal radiance differences between various objects in a scene and display these differences in thermal radiance as a visual image of the scene. Thermal imaging systems are often used to detect fires, overheating machinery, aircraft, vehicles and people, and to control temperature sensitive industrial processes.
The basic components of a thermal imaging system generally include optics for collecting and focusing IR radiation from the scene, an IR detector having a plurality of thermal sensors for converting the IR radiation to an electrical signal, and electronics for amplifying and processing the electrical signal into a visual display and/or for storage in an appropriate medium. A chopper is often included in a thermal imaging system to modulate the IR radiation and to produce a constant background radiance which provides a reference signal. The electronic processing portion of the thermal imagining system will subtract the reference signal from the total radiance signal to produce a signal with minimum background bias.
Thermal imaging systems may use a variety of IR detectors. An IR detector is a device that responds to electromagnetic radiation in the IR spectrum. IR detectors are sometimes classified into two main categories as cooled and uncooled. A cooled IR detector is an IR detector that must be operated at cryogenic temperatures, such at the temperature of liquid nitrogen, to obtain the desired sensitivity to variations in IR radiation. Cooled detectors typically employ thermal sensors having small bandgap semiconductors that generate a change in voltage due to photoelectron interaction. This latter effect is sometimes called the internal photoelectric effect.
Uncooled IR detectors cannot make use of small bandgap semiconductors because the dark current component swamps any signal at room temperature. Consequently, uncooled detectors rely on other physical phenomenon and are typically less sensitive than cooled detectors. However, because uncooled detectors do not require the presence of a cryogenic cooler and associated components, they are less bulky and consume less energy than cooled detectors, they are the preferred choice for portable, low power applications where the greater sensitivity of cooled detectors is not required. In a typical uncooled thermal detector, IR photons are absorbed by the thermal detector and the resulting temperature difference of the absorbing element is detected. Typical uncooled thermal detectors include a pyroelectric detectors, thermocouples and bolometers.
An IR window is a frequency region in the IR spectrum where there is good transmission of electromagnetic radiation through the atmosphere. Typically, IR detectors sense IR radiation in the spectral bands from 3 to 5 microns (having an energy of 0.4 to 0.25 eV) and from 8 to 14 microns (having an energy of 0.16 to 0.09 eV). The 3 to 5 micron spectral band is generally termed the “near IR band” while the 8 to 14 micron spectral band is termed the “far IR band.” IR radiation between the near and far IR bands cannot normally be detected due to atmospheric absorption, although this problem does not arise in those IR imaging systems used in the vacuum of space, which enables the 5–8 micron mid-range IR spectral band to be detected.
The IR scene radiation is typically focused onto a thermal detector by one or more IR lens. IR lenses may be classified as a single field of view lens or as a zoom lens. Zoom lenses, in turn, may be designed to function as a continuous zoom lens or as a two-position zoom lens.
Representative U.S. Patents include U.S. Pat. No. 5,852,516, “Dual Purpose Infrared Lens Assembly Using Diffractive Optics”, Robert B. Chipper, and U.S. Pat. No. 5,973,827, “Refractive/Diffractive Infrared Imager and Optics”, also by Robert B. Chipper.
However, a problem exists in that conventional IR zoom lenses used with uncooled detectors can drift out of focus as the temperature increases or decreases, typically over a range of about −10° C., or less, to about 50° C., or more. The lens drift problem is due in large part to the typically large rate of change of the index of refraction of the lens material as a function of the temperature of the lens material.
Referring to FIG. 1, there is shown a conventional zoom lens assembly 1 having three lenses 2, 3 and 4. The three lenses are each constructed of the same IR radiation transmissive material, or Germanium (Ge) in this case. None of the lens surfaces are diffractive in nature. Lens 3 is movable between a wide field of view (WFOV) and a narrow field of view (NFOV) position by a lens drive assembly 5.
A conventional solution to the temperature-induced focus drift problem involves providing a complex and expensive electro-mechanical assembly 5 to maintain focus over temperature by re-focusing at least the lens 3 within the zoom lens assembly 1. The lens drive assembly may use one or more motors coupled with one or more cams having non-linear motion, and these are combined with motor-driving software that may require look-up tables in order to change the physical location of the lens element 3 over temperature, and as a function of the desired field of view of the lens. This is required as the focusing will typically exhibit a different focus rate versus temperature, depending on the selected zoom position (WFOV or NFOV). In addition, some type of temperature sensor, such as a thermocouple 6, is required to monitor the actual temperature of the zoom lens assembly 1 in order to provide an input to the motor driver software.
As can be appreciated, this complex and costly prior art approach to attempting to maintain focus with the zoom lens assembly is less than desirable.
Reference can also be made to U.S. Pat. No. 5,504,628, “Passive Athermalization of Optics”, by J. F. Borchard. This patent discloses the use of a doublet lens that is optically passively athermalized by choosing two lens materials that have approximately the same Abbe number and substantially different thermal coefficients of refractive index. The ratio of the powers of the lens elements is designed to provide the desired passive athermalization. A diffractive surface is used on one of the lens elements to correct for chromatic aberration. Because the Abbe numbers are approximately the same for the two lens materials, the chromatic correction is said to not significantly change with temperature. This allows the ratio of the powers of the lens elements to control the focal length of the doublet with temperature being independent of chromatic correction.
While it could be argued that one might attempt to use this approach to construct a zoom lens assembly, its use would appear to require six lenses and three diffractive surfaces, i.e., the use of a diffractive doublet in place of each of the prior art lens elements 2, 3 and 4. As can be appreciated, the resulting optical system would require more a complex mechanical mounting arrangement and would have a significantly higher cost.